1. Field of the Invention
The present invention relates in general to air-fuel ratio control systems of an internal combustion engine, and more particularly to air-fuel ratio control systems of a type in which the air-fuel mixture fed to the engine can be controlled to a very lean side in accordance with the surrounding conditions.
2. Description of the Prior Art
Hitherto, in order to improve the fuel consumption, a so-called "lean combustion engine" has been proposed in which the combustion of the engine is carried out with a very lean air-fuel mixture, such as a mixture having an air-fuel ratio of about 20 to 25. In the engines of this type, under low speed and low load condition, the engine operates on such a very lean mixture for the improvement of fuel consumption, while upon requirement of quick acceleration and high torque, a somewhat richer-than-normal mixture is fed to the engine. One of the engines of this type is disclosed in Japanese Patent First Provisional Publication No. 1-187338.
However, due to the inherent construction, even the engines of the above-mentioned lean combustion type have a misfire threshold level in the lean combustion side. That is, when the engine is fed with a lean air-fuel mixture exceeding the misfire threshold level, normal operation of the engine is not obtained. Furthermore, even when the lean air-fuel mixture is somewhat richer than the misfire lean level, the surrounding condition of the engine, such as, the nature of fuel, temperature of surrounding air or the like, tends to cause an unstable combustion of the engine.
Accordingly, hitherto, as is seen from the graph of FIG. 6, the actual lean air-fuel ratio has been set to a level which is considerably richer than the misfire threshold level considering the zone inducing the unstable engine combustion. This means that the lean air-fuel ratio set in the engines of the above-mentioned type fails to provide the engine with a satisfied fuel saving or fuel consumption. Furthermore, the enrichment of the lean air-fuel mixture brings about undesired increase of NOx in the exhaust gas.